Following Claud's question about who wrote the first five books of the Bible (the Pentateuch), here are some links to websites about something called the 'Documentary Hypothesis'. This is a theory someone called Wellhausen first proposed, in which it is suggested that there were four main sources for the Pentateuch. This theory has slightly fallen out of favour/fashion recently but, so far as I'm aware, there aren't any other competing theories that are as well-developed as this one. This doesn't mean that this theory is right, just that it's the fullest suggestion that's out there!
- Wikipedia's entry on the documentary hypothesis (Wikipedia isn't always the most reliable source but can be good for getting an overview of a topic)
- A website about the documentary hypothesis (again this may not be that reliable but it is fairly clear)
- An article strongly criticising the documentary hypothesis
- A book about the making of the Pentateuch
- In this book, on page 27, there's a summary of the state of current scholarship regarding the documentary hypothesis